The present invention relates to a semiconductor device and a method of manufacturing the same, particularly, to improvement in performance of the gate insulating film.
With progress in miniaturization of a MOSFET, the gate electrode is required to have a lower resistance. Also, in a gate electrode using a polycrystalline silicon (polysilicon), the problem of depletion is not negligible nowadays. Therefore, it is of high importance to develop a gate structure consisting of a metal single layer. Concerning the gate insulating film, it is proposed to use a film made of a material having a high dielectric constant, e.g., TiO2, in place of SiO2 for decreasing the equivalent oxide thickness of the gate insulating film.
FIGS. 14A to 14C show a conventional manufacturing process. In the first step, a TiO2 film 502 is formed as a gate insulating film by a LP-CVD method in a thickness of about 10 nm on a silicon substrate 500, as shown in FIG. 14A. Used as the CVD gas is, for example, Ti(C11H19O2)2Cl2. Then, a TiN film 503 in a thickness of 10 to 20 nm is formed on the TiO2 film 502 by a CVD method under a gaseous atmosphere of TiCl4 and NH3 as shown in FIG. 14B. The TiN film 503 acts as a barrier metal layer serving to prevent diffusion of a gate electrode material into the gate insulating film or to control the work function. Further, a metal electrode 504 consisting of W, Al, Cu, etc. is formed by CVD on the TiN film 503, as shown in FIG. 14C.
However, the conventional method described above gives rise to a serious problem. Specifically, it is difficult to form the TiO2 film. 502 having oxygen supplied thereinto completely in the step shown in FIG. 14A. Since the oxygen shortage functions as a donor in the TiO2 film 502, the insulating properties of the TiO2 film are markedly deteriorated by a slight oxygen shortage. It should also be noted that the impurities contained in the CVD gas such as carbon and chlorine are left unremoved so as to cause the oxygen shortage. Further, the CVD film tends to become lower in density than the oxide film formed by thermal oxidation, as can be seen from CVD of a silicon oxide film. As a result, an oxygen shortage tends to be caused. Such being the situation, it is difficult to form a transistor having good characteristics and a high reliability.
On the other hand, the TiO2 film 502 constituting the conventional gate insulating film leaves room for further improvement in the film structure. FIGS. 15A and 15B schematically show the film structure of the TiO2 film 502, wherein FIG. 15A is a cross sectional view, and FIG. 15B is a plan view.
In forming the TiO2 film 502 in the step shown in FIG. 14A, a clear crystal grain boundary 512 is formed between adjacent crystal grains 511 of TiO2, as shown in FIGS. 15A and 15B. As a result, the electrical insulating properties of the TiO2 film constituting the gate insulating film are markedly deteriorated, making it very difficult to prepare a MIS transistor having good characteristics and a high reliability.